Digital data transmitted in an audio/digital communication system, even when transmitted at as much as a 10 KBPS rate, has energy in the audio band (300-3,000 Hz) which cannot be filtered. Digital data transmission bursts, although not at a high volume, can thus cause audio noise which is irritating to communications system users. The transmitter of the communication system mutes the transmitted audio (or voice) when it transmits digital data and accordingly if the receiver of the communication system also mutes its audio circuitry during data transmission times, no meaningful audio signals will be lost.
U.S. Pat. Nos. 4,344,175 to Leslie; 4,020,421 to Elder et al; 3,939,431 to Cohlman; 3,927,376 to Ferrie; 3,909,727 to Hughes et al; and 3,870,958 to Rypinski et al all disclose prior art data muting methods and apparatuses. These and other prior art receiver data muting methods and apparatuses have typically included analog delay circuitry so that after the digital data has been decoded and detected, the delayed audio circuits are then muted. Since the digital data path is not delayed, the analog delay affords the circuitry enough time to decode and mute. These prior art systems are disadvantageous, however, in that they require expensive analog delay circuitry.
Some prior art systems partially decode the digital data (e.g., the synchronization word preceding actual digital data) and then mute, for example see U.S. Pat. Nos. 3,909,727 to Hughes et al, 3,939,431 to Cohlman and 4,344,175 to Leslie. However, these prior systems still allow some initial digital data noise to be heard in the audio circuits, although not to the extent as if no muting was used.
Milliker et al, U.S. Pat. No. 4,430,742 proposes to use a noise detector which responds to detection of a carrier signal so as to mute the transceiver output for a predetermined time interval. If a synchronization word of a data signal is subsequently detected by other circuitry during the relevant predetermined time interval, then the receiver output is further muted until the other circuitry detects the last bit of the following information word, at which time the transceiver output is unmuted. Such a method and apparatus has several disadvantages including the muting of the transceiver whenever the carrier signal is received, whether or not a data signal has been transmitted, and the need to provide complex additional circuitry such as a noise or tone detector, sync word detector, timer, etc.
In the prior art systems, unmuting typically occurs when the end of the digital message has been detected, after a timer times out (the timer being set in accordance with the expected length of the digital message), if the digital data is garbled, etc.